Re: How to clamp a corner frame ?
I don't know what materials your storms are made of. Wood, aluminum and vinyl would each have a different repair method. Glue would really only be appropriate for a wooden storm.
Vinyl has a joint either "welded" (heat-fused) or reinforced with a L insert of nylon plastic. The correct type of cement for the vinyl would be crucial to success. Determine if it's PVC or ABS or other.
Aluminum storms usually have a screw that threads into a channel in the extrusion. These screws could snap or rust and prevent a new screw from being substituted if the stump is still in the way.
Some storms use folded tabs and caulk for the joint. It the tab is broken, replace that type of storm.
Another type of storm uses a cast aluminum L that is crimped into the extrusion. If the casting fails, a repair would require finding or fabricating a new piece.
If it's possible to remove the window sash, it is possible to remove the storm window to the bench for repairs.
Remove not the ancient landmark, which your fathers have set.